Apparatus for measuring conversion of amplitude modulation to phase modulation

ABSTRACT

This apparatus measures conversion of amplitude modulation to phase modulation and is useful where resultant unwanted phase modulation is to be measured, but is mainly intended for testing devices used in 70 MHz frequency modulated communications systems where, for example, the amplitude modulation to phase modulation conversion of limiters and other devices cause distortion of the desired frequency modulated signal.

United States Patent 1 Coackley I UNITED STATES PATENTS Jan. 22, 1974[54] APPARATUS FOR MEASURlNG 3,119,062 l/l964 Codd 324/57 R CONVERSIONOF AMPLITUDE MODULATION TO PHASE MODULATION FOREIGN PATENTS 0RAPPLICATIONS 158,739 3/1964 U.S.S.R.... 324/57 [75] Inventor gg ig gDalgety 206,650 6/1968 U.S.S.R 324 57 73 Ass ee: Hewlett-Packard Ltd.South 1 Queensferry, West Lthian, Primary ExaminerS tanley T.Kravvczewicz Scotland Attorney, Agent, or FirmA. C. Smith [22] Filed:Aug. 24, 1972 [21] App]. No.: 283,602 [57] ABSTRACT This apparatusmeasures conversion of amplitude [30] Forelgn Apphcanolf P nomy Datamodulation to phase modulation and is useful where Oct. 13, 1971 GreatBritain 47,552/71 resultant unwanted phase modulation is to be sured,but is mainly intended for testing devices used [52] US. Cl 324/57 R in70 MHZ frequency modulated communications [51] hit. Cl G01! 27/00 ternswhere, f example, the amplitude modulation to [58] Fleld of Search324/57 R phase modulation conversion of limiters and other vices causedistortion of the desired frequency modu- [56] References Cited latedSignaL 8 Claims, 1 Drawing Figure- 2,416,310 2/1947 Hansen etal...324/57RX OSCILLATOR AMPLITUDE DEVICE UNDER mooumoa TEST com.

u BALANCED PHASE A i OSCILLATOR V MODULATOR SHFTER V men,

ENVELOPE DETECTOR DISCRIMINATOR 1 APPARATUS FOR MEASURING CONVERSION OFAMPLITUDE MODULATION TO PHASE MODULATION BACKGROUND OF THE INVENTIONthesized phase modulated signal is then indicative of the unwanted phasemodulation generated by conversion in the device under test.

Other methods are restricted to the measurement of conversion occurringin limiters since the removal of amplitude modulation is a requirementfor the correct measurement, such as the method described by GR].Baldwin, in The Marconi Review, third quarter 1966, under the title ADirect Method of Measuring AM to PM Conversion in Limiting Amplifiersunder Dynamic Conditions."

SUMMARY OF THE INVENTION According to the present invention there isprovided apparatus for measuring the conversion of amplitude modulationto phase modulation caused by a device under test, including a means forgenerating a carrier frequency, a means for generating a modulatingfrequency, a modulating means having its inputs connected to saidfrequency generators and having its output connected to supply apredetermined known amplitude-modulated signal to the device under testwhich forms part of a test path, phase shifting means forming part of areference path which also includes means for amplitude modification, theamplitude modification and the phase shifting being independent fromeach other, comparing means having its input connected to theoutput ofsaid test path and to the output of said reference path, an envelopedetector connected to said comparing means to indicate the degree ofnulling out achieved by adjustment of the phase shifting and amplitudemodifying means, and a discriminating means having a known frequency tovoltage conversion slope connected to said comparing means.

DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENT An embodiment of thepresent invention will now be described by way of example, withreference to the accompanying drawing, which illustrates the embodimentin diagrammatic form.

An oscillator l is connected to supply a carrier frequency signal tohigh guality amplitude modulator 2, which is also supplied by amodulating frequency signal from a second oscillator 3. The modulator 2is connected to the input of a device under test 4. The same oscillatorsl and 3 are also connected to a balanced modulator 5 which in turn isconnected to a phase shifter 6 forming part of a reference path whichalso includes an attenuator 7.

The outputs of the test path, which includes the device under test, andthe reference path are supplied to an adder or comparator 8 which inturn is connected to an envelope detector 9 and to a discriminator 10.

In operation, the amplitude modulator 2 delivers a predetermined knownamplitude-modulated test signal to the device under test 4 which signalhas virtually no frequency modulated component and practically onlycomprises the carrier frequency and two side bands. The oscillators land 3 simultaneously supply the balanced modulator 5 with the samecarrier and modulating frequencies so that said modulator producesamplitude modulation side bands with a suppressed carrier signal. Thissignal is phase shifted and amplitude modified by the components 6 and 7and is then supplied to the adder 8 together with the output from thedevice under test. The phase and the amplitude of the output signal ofthe reference path are adjusted independently from each other, such thatthe side band components are substantially nulled out as indicated bythe envelope detector 9 which shows a minimum amplitude modulation.However, because of the non-linearities of the device under test therewill remain a certain degree of phase modulation which can be measureddirectly in amplitude using the conventional discriminator 10 with itsknown frequency to voltage slope, since the carrier, not having beennulled out, is still of large amplitude. This provides a low noise highresolution measurement where only phase and amplitude adjustments needbe made.

The discriminator 10 may comprise a limiter to remove small errors dueto any remaining amplitude modulation since at this point the resultantconversion of amplitude modulation to phase modulation can be neglectedwithout introducing an undue error into the measurement.

It will be evident to those skilled in the art, that the describedcircuitry can be modified, for example, the adder may be replaced by asubstractor depending on the phase of the reference signal as modifiedby the phase shifter 6. This phase shifter should preferably have aphase shifting range from 0 to 360. If the device under test has asubstantial delay, it can then become necessary to provide for a delaymeans in the reference channel.

Of course, theattenuation range of the attenuator '7 should be matchedto the dynamic characteristics of the output signal of the device undertest. If this device under test is an active device having a gaingreater than unity there should be also provided an attenuator in thetest path or a respective gain amplifier in the reference path.

As a practical matter to increase the versatility of the instrumentseveral of the mentioned components may be plug-in units.

1. Apparatus for measuring the conversion of amplitude modulation tophase modulation caused by a device under test, including a means forgenerating a carrier frequency, a means for generating a modulatingfrequency, a modulating means having its inputs connected to saidfrequency generators and having its output connected to supply apredetermined known amplitude-modulated signal to the device under testwhich forms part of a test path, phase shifting means forming part of areference path which also includes means for amplitude modification, theamplitude modification and the phase shifting being independent fromeach other, comparing means having its input connected to the output ofsaid test path and to the output of said reference path, an envelopedetector connected to said comparing means to indicate the degree ofnulling out achieved by adjustment of the phase shifting and amplitudemodifying means, and a discriminating means hav ing a known frequency tovoltage conversion slope connected to said comparing means.

2. Apparatus according to claim 1, wherein said reference path comprisesa balanced modulating means the inputs whereof are connected to saidmeans for generating carrier and modulating frequencies, the outputwhereof supplying a signal with amplitude modulation side bandfrequencies but substantially without a carrier frequency component.

3. Apparatus according to claim 1 wherein said phase shifting means hasa phase shifting range from to 360.

4. Apparatus according to claim '1 wherein said amplitude modifyingmeans is an attenuator.

5. Apparatus according to claim 4, wherein said attenuator has anattenuation range from zero to infinite attentuation.

6. Apparatus according to claim 1 wherein said test path furthercomprises an attentuation means to facilitate testing of a device havinga gain greater than unity.

7. Apparatus according to claim 1 wherein said discriminating meanscomprises an amplitude limiting means to suppress remaining amplitudemodulation.

8. Apparatus according to claim 1 wherein said reference path alsocomprises a delay means.

1. Apparatus for measuring the conversion of amplitude modulation to phase modulation caused by a device under test, including a means for generating a carrier frequency, a means for generating a modulating frequency, a modulating means having its inputs connected to said frequency generators and having its output connected to supply a predetermined known amplitudemodulated signal to the device under test which forms part of a test path, phase shifting means forming part of a reference path which also includes means for amplitude modification, the amplitude modification and the phase shifting being independent from each other, comparing means having its input connected to the output of said test path and to the output of said reference path, an envelope detector connected to said comparing means to indicate the degree of nulling out achieved by adjustment of the phase shifting and amplitude modifying means, and a discriminating means having a known frequency to voltage conversion slope connected to said comparing means.
 2. Apparatus according to claim 1, wherein said reference path comprises a balanced modulating means the inputs whereof are connected to said means for generating carrier and modulating frequencies, the output whereof supplying a signal with amplitude modulation side band frequencies but substantiAlly without a carrier frequency component.
 3. Apparatus according to claim 1 wherein said phase shifting means has a phase shifting range from 0* to 360*.
 4. Apparatus according to claim 1 wherein said amplitude modifying means is an attenuator.
 5. Apparatus according to claim 4, wherein said attenuator has an attenuation range from zero to infinite attentuation.
 6. Apparatus according to claim 1 wherein said test path further comprises an attentuation means to facilitate testing of a device having a gain greater than unity.
 7. Apparatus according to claim 1 wherein said discriminating means comprises an amplitude limiting means to suppress remaining amplitude modulation.
 8. Apparatus according to claim 1 wherein said reference path also comprises a delay means. 